A thylakoid is a compartment present inside chloroplasts* and cyanobacteria**. They are the sites responsible for light dependent reactions during photosynthesis. They consist of a thylakoid membrane surrounding a thylakoid lumen. *a plastid in green plant cells which contains chlorophyll. **a bacteria which obtain energy from photosynthesis. A VERY EASY TOPIC IF YOU ARE ABLE TO UNDERSTAND IT.